Application Note 5240 Introduction Many of today's GPS receiver LNA designs are based on discrete transistors rather than using

ALM-1106 ALM-1106 as a 1.575 GHz GPS Low Noise Amplifier Application Note 5240 Introduction Many of today's GPS receiver LNA designs are based on discrete transistors rather than using MMICs because designs using discrete transistors result in amplifiers with lower Noise Figure (NF). Figure 1 shows a typical GPS LNA employing a discrete solution. Obviously, the discrete solution has higher component count, occupying larger PC board area and is generally more difficult to design. In today's portable applications where both circuit compactness and extremely quick time-to-market are required, circuits with discrete transistors are fast becoming less attractive. While it is true that discrete designs still offer the best NF performance, new MMICs provide nearly as good noise performance while offering many benefits such as: An example of such MMIC is the Avago Technologies' ALM-1106 ALM-1106, specifically designed for battery operated GPS LNA application. The ALM-1106 ALM-1106 uses Avago Technologies' proprietary GaAs Enhancement-mode pHEMT process to achieve high gain operation with very low NF, high linearity and is capable of operating under supply condition of down to 1 V. Ca Rbias Rb · Integrated current mirror simplifies the biasing network design. 3V Cb RFC · Internal feedback makes impedance matching easier across a wider bandwidth. · High linearity and low noise are achieved with low current consumption. Ra Ra Rd RFC Input · Enhancement mode FET requires only a single positive supply. TL1 Figure 1. A typical GPS LNA solution with discrete transistor. Output Impedance Matching for Low Noise Operation As the prime consideration in designing this GPS LNA is low Noise Figure, the input matching of the LNA is tuned to achieve lowest possible Noise Figure. In this case, the LNA input return loss takes a backseat. At 1.575 GHz, the 4 Noise Parameters of ALM-1106 ALM-1106 which are opt, Fmin and rn (Normalized Noise Resistance) give values of 0.7147.4°, 0.8 dB and 0.24 respectively. Based on these values, constant Noise Figure circles are plotted and shown in Figure 2. It can be easily seen from Figure 2 that a series inductor followed by a shunt inductor are needed to transform the 50 port impedance to somewhere close to opt in order to minimize the LNA's NF. At 1.575 GHz, the effect of the length of microstrip lines joining the matching inductors and the device input pin should be considered although this does not drastically change the matching circuit topology as commonly seen in high frequency designs. For example, the input series inductor does not transform the 50 port impedance along the 50 constant resistance circle when seen from the position of the following shunt inductor. The effect of the microstrip joining the series and shunt inductors will rotate the transformed impedance clockwise. This is graphically shown in Figure 2. Figure 3 gives a simple input matching circuit that will transform the port impedance close to opt. opt NF=0.85 dB NF=0.90 dB NF=0.95 dB NF=1.00 dB Figure 2. ALM-1106 ALM-1106 constant NF circles at 1.575 GHz. L L1 Term L=5.6 nH Term1 Num=1 R= Z=50 Ohm MLIN TL1 Subst="MSub1" W=21.0 mil L=61.0 mil L L2 L=10 nH R= MLIN TL2 Subst="MSub1" W=21.0 mil L=108.0 mil Term Term2 Num=2 Z=50 Ohm MS ub MSUB MSub1 H=10.0 mil Er=3.8 Mur=1 Cond=1.0E+50 Hu=3.9e+034 mil T=0.7 mil TanD=0.004 Rough=0 mil S -P ARAMETERS S_Param SP1 Start=1.3 GHz Stop=1.7 GHz Step=0.001 GHz Figure 3. An input matching network consisting of a series inductor followed by a shunt inductor to tune the LNA input for low noise. A GPS LNA with ALM-1106 ALM-1106 Figure 4 below shows a typical circuit of an ALM-1106 ALM-1106 configured to operate in the 1.575 GHz GPS band. It is clear that the circuit is more compact compared to the discrete transistor solution shown in Figure 1. Due to the need to maintain a low NFmin, the internal feedback employed in the device is kept to a minimal. In order to maintain unconditional stability, some external damping is necessary as provided by the 12 resistor. The 1.575 GHz GPS LNA is constructed on a 10 mil thick RO4350B RO4350B PCB. The low NF required in the GPS LNA suggests the use of a low loss PCB material to demonstrate the low noise figure achievable with the ALM-1106 ALM-1106. The demoboard PCB is reinforced with an additional layer of FR4 for mechanical strength and rigidity. Figure 5 shows a completed ALM-1106 ALM-1106 demoboard and the board stacking structure is shown in Figure 6. The ALM-1106 ALM-1106 biasing current can be set using external resistor R1. In this application, R1 is empirically determined to be 18 k to set the biasing current to about 10 mA while the LNA supply voltage is +2.85 V. The value of R1 needs to be re-determined when the control voltage driving the SD pin (pin 4) via R1 is not +2.85 V. There are no input and output DC blocking capacitors required in the circuit as the ALM-1106 ALM-1106 incorporates these DC blocking capacitors internally to minimize external component count. R2 12 R1 18 K +2.85 V C2 6.8 pF L3 4.7 nH C1 6.8 pF C3 0.1 mF +2.85 V 4 6 2 L1 5.6 nH 5 ALM-1106 ALM-1106 L2 10 nH Bottom Paddle Figure 4. ALM-1106 ALM-1106 application circuit configured to operate as 1.575 GHz GPS LNA. RO 4350B 4350B 10 mil 44 mil Figure 5. Completed ALM-1106 ALM-1106 demoboard. FR 4 Figure 6. Demoboard PCB layers. copper layers Demoboard layout Figure 7 shows the demoboard PCB layout. Note that C4 was originally included as part of output matching network. It was later found to be unnecessary and is not shown in the schematic in Figure 4. In real application, C4 can be replaced with a short using microstrip. Table 1 shows the list of components required to build the ALM1106 ALM1106 1.575 GHz demoboard. C3 R2 C2 L3 L2 C4 L1 R1 C1 Figure 7. Demoboard layout and component placement. Table 1. Demoboard component list. Component Designator Manufactuer and Part Number C1, C2 6.8 pF 0402 ROHM MCH155A6R8DK MCH155A6R8DK C3 0.1 mF 0603 MURATA GRM40X7R104K25PL GRM40X7R104K25PL C4 100 pF 0402 ROHM MCH155A101JK MCH155A101JK L1 5.6 nH 0402 TOKO LLP1005-FH5N6C LLP1005-FH5N6C L2 10 nH 0402 TOKO LLP1005-FH10NC LLP1005-FH10NC L3 4.7 nH 0402 TOKO LLP1005-FH4N7NC LLP1005-FH4N7NC R1 18 KW 0402 ROHM MCR01J183 MCR01J183 R2 12 W 0402 ROHM MCR01J120 MCR01J120 RF Performance Implementing Shutdown Control The completed demoboard LNA gives approximately 14.5 dB while achieving return loss of better than 10 dB on both input and output port. Figure 9 shows the gain measured on the demoboard while Figure 8 gives the measured input and output return loss. In portable applications where battery power must be conserved, it is desirable that the receiver LNA can be shutdown and switched on under software control. Figure 11 shows a suggestion on how the ALM-1106 ALM-1106 LNA can be placed under software control allowing it to be shutdown and turned on via a microcontroller output port. The suggested scheme here employs a P-Channel Enhancement Mode MOSFET transistor with its Gate connected to a microcontroller output pin. Assuming that the microcontroller is capable of raising its output pin level to that of Vdd (typically +2.85V), the MOSFET transistor can then be turned off when its Gate is at Logic High and thus turning off the ALM-1106 ALM-1106 LNA. A Logic Low turns on the MOSFET switch and in turn switches on the LNA. Since PChannel MOSFETs are available with low RDS(on)(